Natural clays have their structure which must be destroyed during the deformation process . In this paper a new kind of constitutive model capable to describe the damage of soil structure has been proposed. A structured clay in this model is regarded as an assembly of lumps of differentsizes

and it is assumed that the total deformation consists of 3 parts

i.e. elastic deformation of lumps

plastic deformation due to sliding between lumps and irrecoverable deformation due to crushing of lumps. Usually yield function is introduced to describe the plastic deformation

however

anew damage function is proposed here to describe the additional deformation due to the damage of soil structure. Finally

a stress-strain relationship expressed in incremental form is formulated and a procedure is suggested to determine the model constants.